This invention relates to wet oxidation of a fuel and the conversion of released energy into mechanical work. More particularly a fuel is wet oxidized in an inert carrier liquid with the products of oxidation being used to actuate one stroke of a double-acting reciprocating engine. A portion of the heat of oxidation is utilized to raise steam, which in turn is used to actuate the reverse stroke in the double-acting reciprocating engine. In alternate embodiments generated gases are used to drive a turbine and a gas lift pump.
It is well known in the art that materials with a chemical oxygen demand can be oxidized in a wet medium, and that virtually complete oxidation of such materials can be accomplished at temperatures below 610.degree. F. If the material is a solid--such as pulverized coal, wood saw dust or municipal refuse--it is generally preferable to keep the wet medium in the liquid phase so that the material may be kept separated and suspended for reaction with an oxidizer such as oxygen, oxygen enriched air and air. It is also generally preferable that the wet medium be inert to the oxidation reaction as well as to the products of oxidation.
One common wet medium is water which serves satisfactorily within the temperature range for practical wet oxidation, that is, 100.degree. F. to 610.degree. F. At temperatures of 100.degree. F. or lower, the oxidation rate is generally too slow to be of commercial interest in most materials that in effect serve as fuels. At temperatures in the upper end of the range, the use of water as the wet medium tends to become impractical due to the pressures required to keep water in the liquid phase. At temperatures near the upper end of the range, pressures in the order of 3200 psia are required to keep water in liquid form. Pressure vessels required to hold pressures of this magnitude are generally too costly for any practical use of wet oxidation processes.
The importance of keeping the wet medium in the liquid phase for wet oxidation is readily apparent in the wet oxidation of solid fuels. Preferably the fuel is reduced to small particle size and is suspended in the wet medium, for example a slurry. An oxidizer then is introduced into the slurry and the process of wet oxidation is underway with a resultant generation of heat and a corresponding rise in temperature. If the wet medium is water, a modest increase in temperature without pressure restraint will result in the water converting into vapor. The loss of liquid will tend to destroy the slurry with the solids portion forming a cohesive mass that is virtually impervious to the continuation of oxidation at rates of commercial interest. Thus it is apparent that the process should be conducted in a pressure vessel and that the wet medium should be selected from inert liquids that have a critical temperature well above the maximum temperature planned for commercial operations.
We have found that there are several acceptable inert liquids among the family of fluorocarbons containing eight or more carbon atoms, that may be used as the wet medium for wet oxidation processes. These liquids have another desirable attribute in that they readily absorb large volumes of oxygen, and thus provide a vehicle for dispensing oxygen reasonably uniformly in a desirable manner, for example throughout a slurry. Further, these fluorocarbon liquids, once they have given up absorbed oxygen to the wet oxidation reaction, readily absorb more oxygen from an oxygen supply that is added, for example by bubbling a source of oxygen through the liquid fluorocarbon. In this manner the chemical oxygen demand can be met for a fuel suspended in the inert liquid.
Generally, it is preferable for the pressure vessel, sometimes called a reactor, to be in the general configuration of a cylinder with the longest dimension positioned in a vertical direction. In this orientation a substantial column of slurry can be maintained within the reactor with sufficient room for a gas cap in the top portion of the reactor.
The process of wet oxidation can be made continuous by providing means of injecting the slurry which is preferably composed of an inert carrier liquid with suspended solid fuel particles, by providing an oxidizer injection means capable of dispersing oxygen throughout the slurry, by providing an inert liquid withdrawal means near the top of the liquid column, and by providing a gas withdrawal means at or near the top of the reactor. Should the fuel oxidize into an ash residue it is preferable to provide a sludge withdrawal means at or near the bottom of the reactor. Since the wet oxidation process is exothermic, it is also preferable to install a heat exchanger within the reactor at an appropriate position, for example near the top of the liquid column within the reactor.
Many of the fuels of interest for the processes of the present invention are hydrocarbons. Upon wet oxidation, the hydrogen content of such fuels reacts with the oxidizer and forms water. It is preferable that the inert carrier liquid have a specific gravity greater than water so that water formed by reaction will float at the top of the liquid column. With water positioned in this manner, excess quantities may be removed from the reactor by a properly positioned water withdrawal means, and water that flashes to vapor readily becomes a part of the gas cap at the top of the reactor.
It is preferable that the fuel injection means, the inert liquid injection means and the oxidizer injection means be positioned at or near the bottom of the reactor. With the apparatus positioned as described heretofore, continuous operation of the reactor may be attained by balancing fluid injection rates with fluid withdrawal rates. Temperatures within the reactor may be controlled within planned limits by the use of the heat exchanger. Generally the temperatures should be controlled within the reactor within the range of 100.degree. F. and the pressures should be controlled within the range of 30 psia to 3200 psia. For commercial application, however, the temperatures and pressures should be controlled in much narrower ranges, for example temperatures in the range of 250.degree. F. to 500.degree. F. and pressures in the range of 150 psia to 350 psia. In these narrower ranges in some cases water formed as a product of reaction will be in the liquid phase and in other cases water will be in the vapor phase. It is important that the inert liquid be selected so that in all cases of temperatures and pressures the inert liquid will remain in the liquid phase.
Preferably the heat receptive fluid injected into the heat exchanger is water which in turn is removed from the heat exchanger as steam. In the mode as described heretofore two fluids may be withdrawn from the reactor for further useful work; the gaseous products of reaction from wet oxidation and the steam from the heat exchanger. Both fluids contain a considerable amount of energy that can be converted into other forms. For example the fluids can be discharged through a turbine or through a reciprocating engine to accomplish mechanical work. Those of ordinary skill in the art can envision other useful purposes for the energy contained in the discharge fluids, including the heat contained in the inert liquid as well as in any liquid water withdrawn from the top of the liquid column within the reactor.
A wide variety of fuels may be used in the reactor with much of the available energy diverted to useful work in an adjacent engine, the combination of such apparatus sometimes being called a wet oxidation engine. The fuels may be common liquids such as petroleum derivatives or they may be solids such as coal, wood products, plastics and the like. Of particular interest as a fuel is municipal refuse--garbage and rubbish--which is accumulating at alarming rates in and around population centers.
Municipal refuse is generally not regarded highly as a fuel because of the wide variations in its content from batch to batch. Garbage, for example, has a relatively low combustible content generally ranging from 12% to 33% with relatively high moisture content generally ranging from 60% to 85%. In the United States, with widespread usage of household and restaurant waste food grinders, most of the garbage is disposed of through the sewer system rather than accumulating for periodic collection. As a result most of the municipal refuse is composed of rubbish--paper, rags, wood, plastics, glass and metals. It is not unusual in the United States to find municipal refuse that over long periods of times averages 80% combustibles, 13% noncombustibles and 7% ash. With rising costs of energy, the heat value of municipal refuse can be a viable alternate source of energy.
In the United States the two common disposal methods for municipal refuse are by interment in a so-called sanitary land fill and by incineration. In the land fill procedure destruction of the refuse is delayed over a long time period with the resultant underground generation of carbon dioxide and methane. Migration of the methane can cause an unplanned hazardous situation when explosive concentrations accumulate in storm sewers and other void spaces underground. In the incineration procedure refuse is set afire and reduced to ash. The hetrogeneous nature of refuse considerably complicates the problem of complete combustion and great care is required to avoid release of objectionable odors and particulate matter. Considerable improvement over both methods of disposal can be accomplished by wet oxidation of refuse to residual ash, as will be more fully described hereinafter.
The heat content of municipal refuse typically averages in the range of 7,000 to 9,000 BTU per pound on a dry basis. On an as-received basis the average heat content approximates 4,000 BTU/Lb. While the components of refuse vary widely from batch to batch, the chemical composition of combustible material approximates C.sub.6 H.sub.10 O.sub.5. Manual sorting and segregation of refuse into combustibles and noncombustibles is an unpleasant and costly task. Therefore, it is highly desirable to proceed with refuse disposal in an environmentally acceptable way without the manual sorting step.
With approximately 4,000 BTU per pound of heat available from refuse, on an as received basis a portion of this available energy can be applied to proper preparation of the material for destruction. The balance of the available energy can then be directed to further useful work. It is well known in the art how to shred and compact loose bulky material, and no useful purpose is accomplished by detailing such procedures herein. For the methods of the present invention it is preferred that the refuse be reduced in size to units of compact material with maximum dimension of approximately one-half inch. Such sizing and compaction may be accomplished by any convenient method and it is preferable that any liquid residue be disposed of separately. The resulting compacted units of residue will be relatively free of moisture and will be composed of combustibles and noncombustibles.
In some cases it is desirable that substantially all of the moisture be removed from the rubbish with the resultant increase in BTU content per pound of rubbish. Methods of drying are well known in the art, with the choice of methods being dictated in part by economic considerations.
One method of interest involves the displacement of water by an oily fluid. Preferably the oily fluid would be the inert carrier liquid used in the reactor of the present invention.
As mentioned heretofore many fuels can be used in the methods of the present invention. For illustrative purposes the fuel described is compacted municipal refuse. The methods of the present invention can be more clearly understood by reference to the drawings.